Thick film dielectric electroluminescent displays have been developed and are described for example in Applicant's U.S. Pat. No. 5,432,015 (the entirety of which is incorporated herein by reference). Such a display is constructed on a ceramic or other heat resistant substrate. The fabrication process for the display entails first depositing a set of row electrodes on the substrate followed by the deposition of a thick film dielectric layer. Following this, a thin film structure is deposited that comprises one or more thin film dielectric layers sandwiching one or more thin phosphor films and a set of optically transparent column electrodes is deposited using vacuum techniques as exemplified by Applicant's U.S. patent application Ser. No. 09/540,288 filed Mar. 31, 2000 (the entirety of which is incorporated herein by reference). The entire structure is covered with a sealing layer that protects the thick and thin film structures from degradation due to moisture or other atmospheric contaminants.
The composite dielectric layers used in these displays have a high dielectric constant, allowing the use of relatively thick dielectric layers in the displays without a significant increase in the display operating voltage. The relatively thick dielectric layer, typically greater than 10 μm, is used to prevent dielectric breakdown during display operation. Typically, the thick film layer comprises a sintered perovskite piezoelectric or ferroelectric material such as lead magnesium niobate (PMN) or lead magnesium titanate-zirconate (PMN-PT) with a dielectric constant of several thousand. There may also be applied a thinner overlayer of a compatible piezoelectric or ferroelectric material such as lead zirconate titanate (PZT) using metal organic deposition (MOD) or sol gel techniques to smooth the thick film surface for deposition of a thin film phosphor structure.
The fabrication of large area thick film electroluminescent displays for television applications requires the use of low cost substrates that can be fabricated in large areas and maintain their dimensional stability during heat treatment steps used in the display fabrication process. Thick dielectric displays are traditionally constructed on a ceramic substrate that will withstand processing temperatures up to at least 850° C. However, such substrates are typically manufactured using a tape casting process whereby a slurry is prepared comprising the ceramic material and a minor proportion of a sintering aid in powder form in a solvent based liquid vehicle containing a polymeric binder. The slurry is spread on a flexible carrier film, and is then heated to a relatively low temperature to drive off the solvent, thereby forming a flexible “green tape” that is peeled from the carrier film and then sintered at very high temperature on a refractory substrate to form the ceramic substrate. During the sintering process, shrinkage of the tape cast deposit occurs and some distortion and surface roughness of the sintered substrate will result if great care is not exercised in the sintering process. This distortion becomes a greater problem for the large area substrates required for large displays suitable for television.
As a result of the difficulties encountered in manufacturing large-area ceramic substrates, it is desirable to use glass substrates for the displays that can be manufactured using glass manufacturing techniques that do not involve sintering of green tapes. However, glass substrates have lower melting points than ceramic substrates, and so the maximum permissible temperature for manufacturing displays on glass substrates is also lower. This requires phosphor films that can be processed at lower temperatures without loss of performance and stability.
Typically, phosphor materials used in electroluminescent displays, particularly those giving blue light emission, must be annealed at temperatures of about 700° C. or higher to realize the required luminance for television application. By contrast, the maximum processing temperature for displays constructed on a glass substrate to prevent softening and consequent distortion of the substrate is about 650° C. and preferably about 600° C. Thick film dielectric layers have been developed that can be fired at a temperature at or below 650° C. as exemplified in the Applicant's U.S. provisional patent application 60/341,790 filed Dec. 21, 2001 (the entirety of which is incorporated herein by reference). Red and green phosphor materials have also been developed that can be processed at temperatures below 650° C. However, blue phosphors that can deliver the performance required to compete with CRTs for a television application must be processed at temperatures higher than this to achieve acceptable luminosity under feasible operating conditions.
It is therefore desirable to provide blue light-emitting phosphor materials and structures for use in thick film dielectric electroluminescent displays that can be processed at lower temperatures while still maintaining acceptable luminosity.